The present application has been at least partially funded by the Office of Naval Research contract numbers PE 61153 N and N00014-93-1-1126.
1. Field of the Invention
The present invention relates generally to thermoacoustic refrigerators and, more specifically, to a thermoacoustic refrigerator having a relatively small size which utilizes one or more piezoelectric drivers to generate high frequency sound within a resonator. The interaction of the high frequency sound with one or more stacks create a temperature difference across the stack which is thermally anchored at each end to a pair of heat exchangers located on opposite sides of each stack.
2. Background of the Invention
Since the discovery by Merkli and Thomann that cooling can be produced by the thermoacoustic effect in a resonance tube, research has concentrated on developing the effect for practical applications. One approach in the art has been to increase the audio pumping rate. While the experiments of Merkli and Thomann used frequencies of around 100 Hz, Wheatley et al. successfully raised the operating frequency to around 500 Hz and achieved impressive cooling rates in their refrigerator. This has encouraged others to build various configurations of thermoacoustic refrigerators.
An important element in the operation of a thermoacoustic refrigerator is the special thermal interaction of the sound field with a plate or a series of plates known as the stack. It is a weak thermal interaction characterized by a time constant given by xcfx89xcfx84≈1 where xcfx89 is the audio pump frequency and xcfx84 is the thermal relaxation time for a thin layer of gas to interact thermally with a plate or stack. The amount of gas interacting with the stack is determined approximately by the surface area of the stack and by a thermal penetration depth xcex4k given by:
xcex4k=(2K/xcfx89)xc2xd
Here K represents the thermal diffusivity of the working fluid. By increasing xcfx89, the weak coupling condition is met by a reduction of xcex4k and hence of xcfx84. The work of acoustically pumping heat up a temperature gradient as in a refrigerator is essentially performed by the gas within approximately the penetration depth. The amount of this gas has an important dependence on the frequency of the audio drive. In a high frequency refrigerator, smaller distances and masses are utilized thus making the heat conduction process relatively quick.
Each of the prior art thermoacoustic refrigerators are relatively complicated to manufacture and thus expensive. In addition, thermoacoustic refrigerators known in the art tend to be massive and typically not well suited for use on a very small level such as for use in cooling semiconductors and other small electronic devices or biological samples. Thus, it would be advantageous to provide a thermoacoustic refrigerator that can be made relatively small with a fast response time while retaining good cooling abilities. In addition, it would be advantageous to provide a thermoacoustic refrigerator that operates relatively efficiently and that is relatively simple and economical to manufacture.
In accordance with the principles of the present invention, a high frequency thermoacoustic refrigerator is provided. Preferably, the thermoacoustic refrigerator operates at a frequency of at least 4,000 Hz. Utilizing a driver that operates at a high frequency allows the device to be made smaller in size as the wavelength at such a frequency is short. Thus, it is a principle object of the present invention to provide a compact thermoacoustic refrigerator in which its dimensions scale with the wavelength of the audio drive.
The present invention provides a thermoacoustic refrigerator which produces relatively large temperature difference across the stack to attain correspondingly relatively low refrigeration temperatures.
The present invention also provides a thermoacoustic refrigerator that utilizes large temperature oscillations with small displacements along the s tack leading to a large critical temperature gradient across the stack in a thermoacoustic refrigeration.
The present invention further provides a thermoacoustic refrigerator that can operate in the ultrasonic range.
The present invention also provides a thermoacoustic refrigerator that is simple and inexpensive to manufacture and is relatively compact.
The present invention also provides a thermoacoustic refrigerator that is well-suited for working gas high pressure operation.
The present invention further provides a thermoacoustic refrigerator that can be easily adapted for miniaturization.
The present invention also provides a thermoacoustic refrigerator that has a quick response and fast equilibration rate for electronic device heat management.
The present invention further provides a thermoacoustic refrigerator that utilizes a convenient frequency range for a piezoelectric driver since such drivers are relatively light, small, efficient, and inexpensive.
The present invention also provides a thermoacoustic refrigerator in which some components, such as heat exchangers and stack, can be fabricated using photolithography, MEMS, and other film technologies.
The present invention also provides a thermoacoustic refrigerator in which the power density of the device can be raised by increasing the frequency and thus reducing its size.
The present invention further provides a thermoacoustic refrigerator that is useful for many applications that require small compact refrigerators, for example to provide a relatively simple, compact, and inexpensive device that can be used for cooling small electronic components and small biological systems.
The thermoacoustic refrigerator is comprised of a resonator that also functions as a housing for an acoustic driver, a stack and a pair of heat exchangers positioned on opposite sides of the stack. The driver is a piezoelectric or other similar device that can operate at high frequencies of at least 4,000 Hz. The stack may be formed from random fibers that are comprised of a material having poor thermal conductivity, such as cotton or glass wool or an aerogel but with a relatively large surface area. The heat exchangers are preferably comprised of a material having good thermal conductivity such as copper. Finally, the resonator contains a working fluid, such as air or other gases at 1 atmosphere or higher pressures.
A compact thermoacoustic refrigerator in accordance with the principles of the present invention includes an elongate resonator defining a generally cylindrical chamber having first and second closed ends and having a length approximately equal to xc2xd the wavelength of sound produced by the driver.
In one embodiment, a thermoacoustic refrigerator has a length that is adjustable for tuning purposes as with a mechanism for moving one or both ends of the chamber closer to or further away from each other and/or a moving mechanism for positioning the stack-heat exchanger assembly within the chamber.
In another embodiment, a thermoacoustic refrigerator in accordance with the principles of the present invention includes a housing comprised of individual segments or portions that are comprised of materials having relatively high thermal conductivity. These portions are spaced by segments or rings (in the case of a cylindrical housing) that thermally isolate adjacent section from each other. Each thermally isolated section is in contact with one heat exchanger contained therein such that as a heat exchanger changes in temperature, that change is conducted through the associated segment.
In yet another embodiment of the present invention, a thermoacoustic refrigerator includes a resonator which defines a generally cylindrical chamber having a length approximately equal to xc2xd wavelength of sound produced by an associated driver. A second stack is preferably disposed between a first stack and the second end of the resonator opposite the driver. With such a configuration, the first stack will produce a first temperature differential and the second stack will produce a second temperature differential by which the combined change in temperature can be used to raise its efficiency. The same applies to higher mode resonators (e.g., 1 wavelength, 1xc2xd wavelength, 2 wavelength, etc.).
In another embodiment of the present invention, a thermoacoustic refrigerator includes a first driver located at one end of the resonator and a second driver located at an opposite end of the resonator. A plurality of stacks are located at optimal locations within the resonator depending upon the location of the standing waves within the resonator.
In still another embodiment, such a thermoacoustic refrigerator includes two stacks, one located proximate the first driver and a second stack located proximate the second driver. The stacks are located at the location of maximum cooling efficiency within the resonator as determined by the standing wave within the resonator generated by the drivers.
In still another preferred embodiment of a thermoacoustic refrigerator of the present invention, the thermoacoustic refrigerator is provided with multiple stacks inside the resonator, each stack located within the resonator to achieve the greatest temperature difference across each stack. The location of each stack corresponds to a particular location relative to the standing wave generated within the resonator by the pair of drivers.
In another embodiment of the present invention, a thermoacoustic refrigerator is comprised of a rectangularly-shaped resonator, a driver and a pair of stacks located at optimum locations within the resonator to attain the highest temperature difference across the stack.
In another embodiment of the present invention, a thermoacoustic refrigerator is comprised of a rectangularly-shaped resonator, a pair of drivers located in proximate the center of the resonator and facing in opposite directions, and a pair of stacks for each driver positioned on opposite ends of the resonator.
In still another embodiment of the present invention, a method of cooling utilizing thermoacoustic technology comprises providing a sealed elongate chamber with first and second heat exchangers disposed therein and a random fiber stack thermally coupled to the heat exchangers. High frequency sound is generated within the sealed chamber which causes a standing wave in the chamber. A corresponding heat flow from the cold end of the stack to the hot end cooling the cold side heat exchanger and depositing the heat at the hot heat exchanger. By utilizing a chamber having a diameter equal to its length and a random stack material, a mixture of axial, radial and azimuthal resonance modes can be achieved. The radial and azimuthal modes provide thermal mixing in the random stack while the axial mode provides axial heat pumping along the stack between the cold and hot heat exchangers. As the thermoacoustic refrigerators of the present invention are reduced in size, the radial and azimuthal modes help to provide more efficient heat pumping thus increasing the efficiency of the refrigerator.
Since the optimum position of the stack within the chamber resulting in the optimal temperature difference across the stack is a function of the length of the stack in association with the frequency and the wavelength of the sound wave, it may be desirable to allow adjustment of the length of the resonator or adjustment of the position of the stack/heat exchanger unit at the optimal position in the resonator to xe2x80x9ctunexe2x80x9d the resonator or stack/heat exchanger, as the case may be, for maximum efficiency. Thus, the method of cooling further includes adjusting the length of the chamber or positioning the stack and heat exchangers to maximize the temperature difference between the first and second heat exchangers for a given driver.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.